1. Field of the Invention
This invention relates to a method for production of a copolymer of vinyl cyanide compound and aromatic vinyl compound. More particularly, it is concerned with an industrially advantageous method for producing a copolymer of vinyl cyanide compound and aromatic vinyl compound by the suspension polymerization with water as the dispersion medium, the copolymer having high content of vinyl cyanide compound, being uniform in its composition, and having excellent transparency, gas barrier property, and chemical resistant property.
2. Description of the Prior Art
The copolymer of vinyl cyanide compound and aromatic vinyl compound having a uniform composition is excellent in its transparency, heat stability and mechanical properties; in particular, when the copolymer contains therein the vinyl cyanide compound at a high ratio, it exhibits extremely superior characteristics in its gas barrier property and chemical resistant property, in addition to the above-mentioned properties, hence there has been expected development of an industrially advantageous method for producing the copolymer of vinyl cyanide compound and aromatic vinyl compound having a uniform composition and a high content of the vinyl cyanide compound.
As has been well known, the composition of the copolymer to be produced depends on the compositional ratio and reactivity ratio of the monomers to be used in the reaction system. Accordingly, when the copolymerization reaction is caused to proceed from the reaction system of a certain monomer compositional ratio, the compositional ratio of the unreacted monomer and the composition of the copolymer to be produced from it varies with advancement in the copolymerization reaction, except for a case wherein the monomer compositional ratio is of an azeotropic composition. In the case of producing a copolymer of acrylonitrile and styrene, which is one example of the copolymer of vinyl cyanide compound and aromatic vinyl compound, by the suspension polymerization method, when the compositional ratio of acrylonitrile and styrene at their initial charging is set at more than 24% by weight (the azeotropic composition) of acrylonitrile (e.g. 96% by weight), there is obtained the copolymer at the initial stage of the copolymerization reaction with the acrylonitrile content of 65% by weight. However, from the relationship in the reactivity ratio of the monomers, a ratio of the unreacted styrene with respect to the unreacted acrylonitrile gradually decreases with advancement of the copolymerization reaction. As the consequence of this, the copolymer to be produced at the last stage of the copolymerization reaction would inevitably have a high content of acrylonitrile, hence the compositional distribution of the copolymer to be resulted from a series of the copolymerization reaction becomes very broad. In addition, since a copolymer having such broad compositional distribution has poor compatibility among the constituent elements with the result that its transparency becomes inferior with accompaniment of other disadvantages to be derived from non-uniformity in the constituent elements.
From the above-mentioned background, in order to produce the copolymer having a uniform composition as intended and excellent quality, it is important that the compositional ratio of the monomer compound as at the start of the copolymerization reaction be maintained until termination of the reaction by first carrying out the initial charging of the constituent elements at a monomer compositional ratio adapted to the reactivity ratio of the monomers, followed by initiation of the copolymerization reaction, and thereafter, as the copolymerization reaction proceeds, supplementing the monomer compound of quick consumption into the reaction system.
In the suspension polymerization method, there have so far been proposed several methods for producing the copolymer of vinyl cyanide compound and aromatic vinyl compound of a relatively high content of the vinyl cyanide compound by additional feeding of the aromatic vinyl compound into the reaction system.
For instance, Japanese Examined Patent Publication No. 36315/1976 (Counter Part: U.S. Pat. No. 4,039,734) discloses a method, in which the aromatic vinyl compound is supplied in accordance with the amount of heat found from detection of the reaction heat during the copolymerization reaction. This method, however, has disadvantages such that the operations for the copolymerization reaction are extremely complicated, and others, because errors in measurement of the reaction heat is great, the copolymer of non-uniform composition is prone to be readily yielded, and, moreover, the feeding quantity of the aromatic vinyl compound varies at every hour.
Further, U.S. Pat. specification No. 4,088,811 discloses a method, in which the copolymerization reaction is conducted at a relatively low temperature of from 70.degree. C. to 90.degree. C. by use of a polymerization initiator such as azobisisobutyronitrile, etc., while continuously supplementing the aromatic vinyl compound. However, this method has also disadvantages such that, depending on the polymerization apparatus to be used, it is difficult to maintain the reaction system at a temperature ranging from 70.degree. C. to 90.degree. C. against the heat of reaction, for which a special heat-removing method has to be adopted; and, in the stripping step for the residual monomer subsequent to the polymerization step, the temperature is usually elevated to a range of from 130.degree. C. to 150.degree. C., which might take a long time until it attains the required stripping temperature, when the polymerization temperature is low, in other words, a waiting time for the elevated temperature becomes prolonged to result in reduced production efficiency; and so forth.
Besides the above-mentioned methods, there have been proposed various methods such as, for example, one, in which the copolymerization is conducted by stepwise setting of the reaction temperature for every conversion (Japanese Examined Patent Publication No. 27808/1971); one, in which the bulk polymerization and the suspension polymerization are combined (Japanese Examined Patent Publication No. 37836/1974); one, in which a plurality of particular polymerization initiators are combined and the temperature is elevated stepwise (Japanese Examined Patent Publication No. 33917/1975), and others. All these methods, however, can hardly be said to be satisfactory as the methods for producing, with industrial advantage, the copolymer of vinyl cyanide compound and aromatic vinyl compound having a uniform composition and a high content of vinyl cyanide compound.